1. Field of the Invention
The invention relates to semiconductor methods and micromechanical structures and, particularly, to a method for forming gaps in a micromechanical device and a micromechanical device with the gaps.
2. Description of Related Art
Micromachining technology, and in particular, microelectromechanical systems (MEMS) technology, is usually applied to size gaps in the microscale device to allow it to respond to mechanical stimulation. In the microscale device, gaps are usually formed between two different material layers. Typically, a sacrificial layer is employed to form such gaps. In practice, the sacrificial layer is deposited between two different material layers, and then portions of the sacrificial layer are removed by etching, such as wet etching to form gaps.
Wet etching is chemical etching in which a substrate is immersed in a bath of etchant. Because the materials that can be dissolved by the etchant will be removed and the other materials will not be damaged, a gap is formed between layers. Thus, wet etching offers advantages of high selectivity. However, etchants employed by wet etching are usually isotropic, that is, they erode material equally in all directions. Therefore, a pattern fabricated by wet etching is usually not easily controlled.
Dry etching refers to the removal of materials by exposing the material to ion bombardment. Unlike wet etching, dry etching typically etches anisotropically, considerably preferable to modern processes because of sharp, well-controlled features. However, dry etching has no selectivity for etching materials and process is complicated causing difficulties for mass production.
With improvement of micromachining technology, there is an ongoing demand for microscale devices with thin and light structures to improve performance. Consequentially, there is a need for manufacture of equally scaled gaps in devices. However, some problems occur when etching thinner sacrificial layers by conventional wet or dry etching. The precision of microscale structure is difficult to satisfy with wet etching. In addition, under such conditions, two material layers at opposite sides of the sacrificial layer are more likely to adhere to each other if a thinner sacrificial layer has been etched. Alternatively, because dry etching such as reactive ion etching (RIE), normally utilizes high energy, materials other than those of the sacrificial layer will be damaged by ion bombardment. Namely, such unexpected failures can create defects in the micrometer device.
What is needed, therefore, is a method for forming gaps in a micromechanical device and a micromechanical device that provide simpler process and avoid the unexpected failures discussed.